1. Field of the Invention
The present invention concerns a computed tomography system (CT system) of the type having at least one rotor in a gantry that in operation can be rotated around a system axis, on which rotor side is arranged at least one x-ray tube cooled by a liquid cooling system equipped with a liquid volume filled with coolant liquid, which liquid volume extends over distances of varying length from the system axis and is located on the rotor of the gantry that is exposed to centrifugal force in operation.
2. Description of the Prior Art
In x-ray tubes more than 99% of the energy used is converted into heat in the generation of x-ray radiation. This heat is discharged from the x-ray tube with the use of a coolant. In x-ray tubes for computed tomography the coolant circuit is subjected to centrifugal forces at the rotor of the gantry that rotates in the CT system, since the at least one x-ray tube is also arranged at the gantry, so that pressure differences arise in the cooling system. The lowest pressure is at the point in the radiator system that is located at the smallest radius in the CT system, and thus is exposed to the lowest centrifugal forces. The exit window of the x-ray tube (which must be cooled particularly well) is typically located optimally close to the patient or to the rotation center of the gantry in order to provide sufficient dose output. Water or anticorrosion or antifreeze agents are normally used as coolants to cool the x-ray tube, as well as the exit window of the x-ray tube. If the coolant temperature in the region of high heat transfer rises to the boiling point, the fluid vaporizes and the heat transport is interrupted. The boiling temperature depends on the pressure, such that higher temperatures can also be allowed at a higher pressure without boiling of the coolant fluid occurring. However, a pressure gradient exists in the rotating gantry due to the centrifugal force increasing away from the rotation center, and decreasing toward the rotation center, such that the pressure near the rotation center (thus in the region of the exit window that is severely thermally charged) is particularly low. A tendency toward bubble formation in this region can be counteracted only by measures to increase pressure.
An additional problem is that expansion occurs with the heating of the coolant, such that an expansion vessel or measures with similar function are necessary in order to prevent destruction of the cooling system due to the expansion of the coolant.
In principle, two different cooling system technologies are known, namely pressure-less systems, as are described in the WO2007/127939 A2. For example, in such a system a membrane is used that is shifted upon expansion of the coolant medium. However, in this technique the pressure of the system identical to the environment pressure, which leads—as described above—to relatively low boiling temperatures, or makes it necessary to use special coolants with a high boiling point. Another embodiment of the cooling system for x-ray tubes in CT systems are known as closed (sealed) cooling systems, in which a boundary surface is charged with a force (for example a mechanical elastic tension or a pressure from a sealed gas volume or shock absorber), as described in U.S. Pat. No. 7,221,736 B2, for example. Due to the expansion of the coolant with increasing temperature, the counter-force at the membrane is increased and the pressure in the system likewise rises with increasing temperature.
A problem in the last variant of the known closed cooling system is that a new adjustment of the pressure by service technicians with special tools is necessary after exchanging a part of the cooling system (for example the radiator or the heat exchanger), and achieving a correct setting proves to be relatively difficult and has a certain tendency to error.